Non-invasive physiological monitoring is a common means for testing, detecting, and treating a physiological condition. Typically, non-invasive monitoring techniques such as pulse oximetry, electrocardiography (ECG), electroencephalography (EEG), and ultrasonic imaging, to name a few, require that a sensor be placed in direct contact with a patient undergoing the procedure.
Pulse oximetry involves the non-invasive monitoring of oxygen saturation level in blood-profused tissue indicative of certain vascular conditions. In practice, light is passed through a portion of a patient's body which contains arterial blood flow. An optical sensor is used to detect light which has passed through the body, and variations in the detected light at various wavelengths are then used to determined arterial oxygen saturation and/or pulse rates. Oxygen saturation may be calculated using some form of the classical absorption equation know as Beer's law.
Accurate measurement of oxygen saturation levels are predicated upon optical sensing in the presence of arterial blood flow. A finger provides a convenient access to a body part through which light will readily pass. Local vascular flow in a finger is dependent on several factors which affect the supply of blood. Blood flow may be affected by centrally mediated vasoconstriction, which must be alleviated by managing the perceived central causes. Peripheral constriction via external compression, however, can be induced by local causes. One such cause of local vasococompression is the pressure exerted by the sensor on the finger. Many currently available pulse oximetry finger sensors have a hard shell which has a high profile and is maintained on the finger by the action of a spring. Since excess pressure on the finger can dampen or eliminate the pulsation in the blood supply to the finger, these springs are intentionally relatively weak. The result of this compromise is that the spring-held sensors readily fall off the finger. It is desirable for a finger sensor to be retained on the finger with only slight pressure, while at the same time being immune to easy dislocation.
Non-disposable finger sensors typically utilize a clamp design for retaining the sensor on the finger. Such devices generally consist of a small spring-loaded clip which attaches to the finger tip in a manner similar to a common clothespin.
Many known non-disposable sensors are relatively bulky. The prior art sensors with their high profile exhibit a relatively high inertia of the housing relative to the finger. This results in a susceptibility to relative motion between the sensor and the finger as the finger is moved. This relative motion manifests itself as motion artifacts in the detected signal. It would be desirable for a finger sensor to be as light as possible so as to minimized relative inertial motion between the sensor and the finger.
Motion artifacts caused by displacement of the lead wire are especially problematic for oximetric sensors. Common oximetric finger sensors often locate the lead wire from the sensor over a central portion of a patient's finger. When the patient flexes or curls his finger, it is common for the lead wire to pull against the sensor causing the light elements to be displaced.
Consequently, there is a need in the art for a sensor assembly which is capable of mechanically isolating a sensor holder without the need to tightly secure the sensor to the patient.